The invention relates to a protection system in a power modulator for the protection of a load connected to the power modulator.
A power modulator is an apparatus supplying power to technical high-power impulse components, which are connected to the power modulator. Such components are, for example, a klystron or a high power transmitter tube, which are very expensive components that must be protected from destructive operations such as an excessive internal voltage breakdown.
For the generation of high-power pulses energy storage devices are required, which store the energy required for such pulses and transfer it to the load in an extremely short period of time. As energy storage devices, Pulse Forming Networks (PFN) as well as large capacities or inductivities are utilized. The energy varies, depending on applications, from below 1 kJ for example in radar applications up to several MJ in apparatus for nuclear fusion. In any case, the energy, which can be released during a breakdown in the load, for example, in a power klystron must not exceed 20-30 J.
A modulator which utilizes a capacitive storage device has been developed by the Fermi National Accelerator Laboratories (see report on the conference concerning the Twenty First International Power Modulator Symposium, 1994; Jun. 27-30, 1994, Westen Sott Toast, Plasma Hotel, Costa Mesa Calif.; H. Pfeffer et al., 3.3: xe2x80x9cA Long Pulse Modulator for Reduced Size and Costxe2x80x9d). This modulator uses a switched condenser bank of moderate size whose voltage drops during the pulse to about 20% of the initial voltage. The energy stored in the condenser bank is connected to the high voltage side of the input of a pulse transformer by way of a cascade of Gate-Turn-Off Thyristors (GTO).
A method and a circuit for the protection of a linear beam apparatus, such as a klystron, from destruction by a cathode-ground-short-circuit is disclosed in U.S. Pat. No. 3,786,275. Power is supplied to the linear beam apparatus in the ground portion thereof and in the collector-electrode by two power supply units arranged in series, which are both associated with the cathode-electrodes. A diode, which is so connected that the current flow away from the collector-electrode is prevented, is arranged between the collector electrode and the connection between the power supply units. The diode prevents in this way a high-current flow from the collector power supply resulting from a short circuit between the cathode and the ground part.
During the impulse a relatively large amount of energy is stored in the stray inductivity and in the main inductivity of the impulse transformer. The amount of this energy (energy in the stray inductivity and in the main inductivity of the impulse transformer is about the same, ca. 400 J) exceeds the admissible energy limit that may be released during a breakdown by an order of size. During a breakdown in the klystron, the control system supplies a signal to the GTO switch for disconnecting the condenser bank from the impulse transformer as well as the ignition switch which ensures a rapid discharge of the condenser bank by way of a small resistor. In this process, the energy stored in the condenser bank is converted in the resistor into thermal energy.
When the condenser bank is disconnected from the load, the energy stored in the stray inductivity is released almost completely in the special two-pole protection circuit, which is arranged in parallel with the primary coil of the impulse transformer. This means that during normal operation of the modulator, the energy released during a breakdown in the klystron does not exceed the admissible value of 20 J. But during a malfunction of the GTO switch or its control system, the above mentioned protection circuit, which is arranged parallel to the primary coil of the impulse transformer, is bridged by the circuit of the ignition switch. In this case, almost the complete energy stored in the impulse transformer is released in the klystron whereby the klystron may be destroyed.
Similar problems are encountered in modulators with an inductive storage (see the conference contribution presented during the above mentioned Power Modulator Symposium in Costa Mesa, H. Salbert et al., under 3.4: xe2x80x9cGeneration of High Power Pulses Using a SMESxe2x80x9d).
It is the object of the present invention to expand and improve the protection system in a power modulator, which includes as a load for example a klystron, to such a degree that the energy released in the klystron during a breakdown is substantially reduced and remains within acceptable limits even if the protection means provided in present arrangements fail.
In a production system included in a power modulator for the protection of a load connected to the power modulator, comprising a charging system, a power impulse former, a control arrangement and a pulse transformer with a first protection circuit in the form of a crowbar circuit connected to the input side and the load connected to the output side of the pulse transformer, a second protection circuit is connected to the output side of the pulse transformer between the low potential and the ground potential connections thereof wherein the second protection circuit includes a controlled switch with a resistor arranged in parallel therewith.
In the protection circuit provided according to the invention, a fault can occur in principle in both current flow directions. In one embodiment of the invention, protection needs to be provided from a breakdown of the load only in one direction as a diode arranged in the circuit determines the admissible breakdown flow direction. In the opposite direction, the diode provides such a high directional resistance that a high current cannot be established in the blocking direction of the diode.
Based on the components, the power modulator consists of a charging system, a power impulse former, a pulse transformer and the load connected thereto. The additional or supplemental protection circuit is disposed on the load side of the pulse transformer between the low potential connection of the pulse transformer and the ground potential. It comprises a controlled switch, which is bridged by a resistor. The resistor is adapted in its size and protective function to the dimensions of the power modulator. Instead of the resistor alone, a series-circuit including the resistor and a capacitor may be used to bridge the controlled switch wherein the dimensions of the resistor and the capacitor are also adapted to the respective functions.
If a fault is to occur only for one direction of current flow, a diode is arranged in series with the protective circuit and the load. The diode prevents substantial current flow in its blocking direction. The diode may be bridged by a resistor of a predetermined size, which limits the voltage potential across the diode.
If the load of the modulator is a klystron, the anode of the diode is connected to the low-potential connector at the load side of the pulse transformer and the cathode is connected to the contact of the switch, which is not at ground potential, or, equally effective, the diode is disposedxe2x80x94in this orientation xe2x80x94between ground potential and the controllable switch.
The advantage of the modulator designed in this manner resides in that the part of the energy resulting from the impulse transformer, which is released during various fault occurrences in the klystron, is substantially reduced with the use of the described additional circuitry.
for example: during malfunction of the commutator in the discharge circuit of the energy storage device the energy from the stray inductivity of the impulse transformer is discharged by way of the ballast resistor arranged in parallel with the switch;
with the above mentioned diode, furthermore, the likelihood of a repeated breakdown of the klystron during the change of the polarity of the voltage effective thereon is substantially reduced.
If however, there is a repeated breakdown of the klystron during the change of the polarity of the voltage effective thereon, a large part of the energy stored in the main inductivity of the impulse transformer is released, not anymore in the klystron, but in the protection circuit proposed herein and in the supplemental circuit mentioned earlier.
The supplemental circuit furthermore provides for a substantial reduction of the energy to be commuted through the protective switch.
Below the invention will be described in greater detail on the basis of the accompanying drawings.